The present invention relates to a cell balance circuit that achieves a balance of voltages among rechargeable cells to avoid a large difference in voltage among the cells when they are connected in series. The cell balance circuit is particularly suitable for a lithium ion cell.
Lithium ion cells are often used as a cell pack in which two or more cells are connected in series. Such lithium ion cells may have a different voltage. There arises a difference in voltage from cell to cell when two or more cells having different voltages are connected in series. Taking this into consideration, a protective circuit is typically included in the cell pack. The protective circuit inhibits charging of the cell when the voltage of that cell exceeds a first predetermined voltage (which is referred to as an over-charge protecting voltage). The protective circuit also inhibits discharging of the cell when the voltage of that cell drops below a second predetermined voltage (which is referred to as an over-discharge protecting voltage).
With such a protective circuit, the charge is inhibited at a voltage value of the cell having the highest voltage. This means that the cell having a lower voltage is not charged enough. In other words, not all the cells can be charged to have a predetermined amount of electrical energy.
On the other hand, discharging is inhibited at a voltage value of the cell having the lowest voltage. This means that the cell having a higher voltage is not discharged enough. In other words, not all the cells can discharge a predetermined amount of electrical energy. Therefore, both the amounts of charge and discharge are significantly smaller than those in the cell pack having no difference in voltage among the cells.
As one of the measures against the above-mentioned problem, the cells having the same or close voltages are combined to avoid the difference in voltage to assemble the cell pack. However, repeated charging and discharging sometimes cause a difference in voltage between or among the cells. This is because a degree of deterioration differs from cell to cell.
Such a circumstance develops a need for a cell balance circuit that reduces the difference in voltage between or among the cells, if any. The cell balance circuit measures the difference in voltage between the cells and adjusts the amount of charge or discharge by the amount corresponding to the difference in voltage. In this event, a circuit used for measuring the difference in voltage should have a high accuracy of measurement. To achieve this requires a significant amount of cost. The cell balance circuit requiring such a high cost, however, is not practical relative to a possibility of occurrence of such a difference in voltage. In addition, a circuit that adjusts the amount of charge is more complicated than a circuit that adjusts the amount of discharge because the former affects the charging route and the charging circuit.